Servers indicate their support for this extended operation by
providing a whoamiOID object identifier as a value of the
'supportedExtension' attribute type in their root DSE. The server
SHOULD advertise this extension only when the client is willing and
^^^^^^^^^^
able to perform this operation.

It should say:

Servers indicate their support for this extended operation by
providing a whoamiOID object identifier as a value of the
'supportedExtension' attribute type in their root DSE. The server
SHOULD advertise this extension only when it is willing
^^
and able to perform this operation.

Notes:

As far as I can see, the last sentence there is misleading and
does not match the operational scenario; and hence, it should be
clarified. According to the recommendations given, the client
will not try the operation if the OID is not offered by the server,
and hence the server cannot know whether the client is willing
to send the whoami Request; and in this case, the *server* will
perform the operation, i.e., send the whoami Response.